On Mon, Sep 14, 2020 at 09:16:43AM -0700, Nick Desaulniers wrote:

LLVM implemented a recent "libcall optimization" that lowers calls to `sprintf(dest, "%s", str)` where the return value is used to `stpcpy(dest, str) - dest`. This generally avoids the machinery involved in parsing format strings. `stpcpy` is just like `strcpy` except it returns the pointer to the new tail of `dest`. This optimization was introduced into clang-12.

Implement this so that we don't observe linkage failures due to missing symbol definitions for `stpcpy`.

Similar to last year's fire drill with: commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")

The kernel is somewhere between a "freestanding" environment (no full libc) and "hosted" environment (many symbols from libc exist with the same type, function signature, and semantics).

As H. Peter Anvin notes, there's not really a great way to inform the compiler that you're targeting a freestanding environment but would like to opt-in to some libcall optimizations (see pr/47280 below), rather than opt-out.

Arvind notes, -fno-builtin-* behaves slightly differently between GCC and Clang, and Clang is missing many __builtin_* definitions, which I consider a bug in Clang and am working on fixing.

Masahiro summarizes the subtle distinction between compilers justly: To prevent transformation from foo() into bar(), there are two ways in Clang to do that; -fno-builtin-foo, and -fno-builtin-bar. There is only one in GCC; -fno-buitin-foo.

(Any difference in that behavior in Clang is likely a bug from a missing __builtin_* definition.)

Masahiro also notes: We want to disable optimization from foo() to bar(), but we may still benefit from the optimization from foo() into something else. If GCC implements the same transform, we would run into a problem because it is not -fno-builtin-bar, but -fno-builtin-foo that disables that optimization.

In this regard, -fno-builtin-foo would be more future-proof than -fno-built-bar, but -fno-builtin-foo is still potentially overkill. We may want to prevent calls from foo() being optimized into calls to bar(), but we still may want other optimization on calls to foo().

It seems that compilers today don't quite provide the fine grain control over which libcall optimizations pseudo-freestanding environments would prefer.

Finally, Kees notes that this interface is unsafe, so we should not encourage its use. As such, I've removed the declaration from any header, but it still needs to be exported to avoid linkage errors in modules.

Reported-by: Sami Tolvanen samitolvanen@google.com Suggested-by: Andy Lavr andy.lavr@gmail.com Suggested-by: Arvind Sankar nivedita@alum.mit.edu Suggested-by: Joe Perches joe@perches.com Suggested-by: Kees Cook keescook@chromium.org Suggested-by: Masahiro Yamada masahiroy@kernel.org Suggested-by: Rasmus Villemoes linux@rasmusvillemoes.dk Signed-off-by: Nick Desaulniers ndesaulniers@google.com Tested-by: Nathan Chancellor natechancellor@gmail.com

Reviewed-by: Nathan Chancellor natechancellor@gmail.com

It would be nice to get this into mainline sooner rather than later so that it can start filtering into the stable trees. ToT LLVM builds have been broken for a month now.

Cc: stable@vger.kernel.org Link: https://bugs.llvm.org/show_bug.cgi?id=47162 Link: https://bugs.llvm.org/show_bug.cgi?id=47280 Link: https://github.com/ClangBuiltLinux/linux/issues/1126 Link: https://man7.org/linux/man-pages/man3/stpcpy.3.html Link: https://pubs.opengroup.org/onlinepubs/9699919799/functions/stpcpy.html Link: https://reviews.llvm.org/D85963

---

Changes V5:

• fix missing parens in comment.

Changes V4:

• Roll up Kees' comment fixup from https://lore.kernel.org/lkml/202009060302.4574D8D0E0@keescook/#t.
• Keep Nathan's tested by tag.
• Add Kees' reviewed by tag from https://lore.kernel.org/lkml/202009031446.3865FE82B@keescook/.

For the record, I don't see Kees' review tag on this.

Changes V3:

• Drop Sami's Tested by tag; newer patch.
• Add EXPORT_SYMBOL as per Andy.
• Rewrite commit message, rewrote part of what Masahiro said to be generic in terms of foo() and bar().
• Prefer %NUL-terminated to NULL terminated. NUL is the ASCII character '\0', as per Arvind and Rasmus.

Changes V2:

• Added Sami's Tested by; though the patch changed implementation, the missing symbol at link time was the problem Sami was observing.
• Fix __restrict -> __restrict__ typo as per Joe.
• Drop note about restrict from commit message as per Arvind.
• Fix NULL -> NUL as per Arvind; NUL is ASCII '\0'. TIL
• Fix off by one error as per Arvind; I had another off by one error in my test program that was masking this.

lib/string.c | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+)

diff --git a/lib/string.c b/lib/string.c index 6012c385fb31..4288e0158d47 100644 --- a/lib/string.c +++ b/lib/string.c @@ -272,6 +272,30 @@ ssize_t strscpy_pad(char *dest, const char *src, size_t count) } EXPORT_SYMBOL(strscpy_pad); +/**

• • stpcpy - copy a string from src to dest returning a pointer to the new end

• •      of dest, including src's %NUL-terminator. May overrun dest.
    

• • @dest: pointer to end of string being copied into. Must be large enough

• •    to receive copy.
    

• • @src: pointer to the beginning of string being copied from. Must not overlap

• •   dest.
    

• • stpcpy differs from strcpy in a key way: the return value is a pointer
• • to the new %NUL-terminating character in @dest. (For strcpy, the return
• • value is a pointer to the start of @dest). This interface is considered
• • unsafe as it doesn't perform bounds checking of the inputs. As such it's
• • not recommended for usage. Instead, its definition is provided in case
• • the compiler lowers other libcalls to stpcpy.
• */

+char *stpcpy(char *__restrict__ dest, const char *__restrict__ src); +char *stpcpy(char *__restrict__ dest, const char *__restrict__ src) +{

• while ((*dest++ = *src++) != '\0')

• /* nothing */;
  

• return --dest;

+} +EXPORT_SYMBOL(stpcpy);

#ifndef __HAVE_ARCH_STRCAT /**

• strcat - Append one %NUL-terminated string to another

-- 2.28.0.618.gf4bc123cb7-goog